1,411 research outputs found
Helicity Transfer in Turbulent Models
Helicity transfer in a shell model of turbulence is investigated. We show
that a Reynolds-independent helicity flux is present in the model when the
large scale forcing breaks inversion symmetry. The equivalent in Shell Models
of the ``2/15 law'', obtained from helicity conservation in Navier-Stokes eqs.,
is derived and tested. The odd part of helicity flux statistic is found to be
dominated by a few very intense events. In a particular model, we calculate
analytically leading and sub-leading contribution to the scaling of triple
velocity correlation.Comment: 4 pages, LaTex, 2 figure
Extreme events in the dispersions of two neighboring particles under the influence of fluid turbulence
We present a numerical study of two-particle dispersion from point-sources in
3D incompressible Homogeneous and Isotropic turbulence, at Reynolds number Re
\simeq 300. Tracer particles are emitted in bunches from localized sources
smaller than the Kolmogorov scale. We report the first quantitative evidence,
supported by an unprecedented statistics, of the deviations of relative
dispersion from Richardson's picture. Deviations are due to extreme events of
pairs separating much faster than average, and of pairs remaining close for
long times. The two classes of events are the fingerprint of complete different
physics, the former being dominated by inertial subrange and large-scale
fluctuations, while the latter by the dissipation subrange. A comparison of
relative separation in surrogate white-in-time velocity field, with correct
viscous-, inertial- and integral-scale properties allows us to assess the
importance of temporal correlations along tracer trajectories.Comment: 5 pages, 6 figure
Helicity advection in Turbulent Models
Helicity transfer in a shell model of turbulence is investigated. In
particular, we study the scaling behavior of helicity transfer in a dynamical
model of turbulence lacking inversion symmetry. We present some
phenomenological and numerical support to the idea that Helicity becomes -at
scale small enough- a passively-advected quantity.Comment: 6 pages, 2 figures, contribution to the proceedings of the
conference: Disorder and Chaos, in honour of Giovanni Paladin, September
22-24, 1997, Rom
On the Heat Transfer in Rayleigh-Benard systems
In this paper we discuss some theoretical aspects concerning the scaling laws
of the Nusselt number versus the Rayleigh number in a Rayleigh-Benard cell. We
present a new set of numerical simulations and compare our findings against the
predictions of existing models. We then propose a new theory which relies on
the hypothesis of Bolgiano scaling. Our approach generalizes the one proposed
by Kadanoff, Libchaber and coworkers and solves some of the inconsistencies
raised in the recent literature.Comment: 10 pages, 5 figure
How to detect illegal waste shipments? The case of the international trade in polyethylene waste
The purpose of this research is to provide a methodological framework that is able to enhance our capability to detect
illegal waste shipment with particular reference to waste plastics. Based on a very large cross-sectional dataset
covering 187 countries over the period 2002-2012, our study aims to do this by using both the mirror statistics method
and the network analysis. Specifically, by using mirror statistics, we identify the existence of a set of “suspicious”
trade relations between pairs of countries. Then, we employ social network analysis in order to define the position of
each country in this illegal trade structure, and to have a clear exposition of the connections between them. Our main
findings reveal the central positions of the USA, Germany and the UK as sources and China and Malaysia as outlets of
illegal shipments of waste plastics. Moreover, our methodology allows us to highlight the presence of other countries,
which carry out an intermediary role within the global trade network, and to detect the changes in traditional illegal
shipment routes. Therefore, this paper shows how social network analysis provides a useful instrument by means of
which crime analysts and police detectives can develop effective strategies to interdict criminal activities
Lattice Boltzmann simulations of droplet dynamics in time-dependent flows
We study the deformation and dynamics of droplets in time-dependent flows
using 3D numerical simulations of two immiscible fluids based on the lattice
Boltzmann model (LBM). Analytical models are available in the literature, which
assume the droplet shape to be an ellipsoid at all times (P.L. Maffettone, M.
Minale, J. Non-Newton. Fluid Mech 78, 227 (1998); M. Minale, Rheol. Acta 47,
667 (2008)). Beyond the practical importance of using a mesoscale simulation to
assess ab-initio the robustness and limitations of such theoretical models, our
simulations are also key to discuss - in controlled situations - some relevant
phenomenology related to the interplay between the flow time scales and the
droplet time scales regarding the transparency transition for high enough shear
frequencies for an external oscillating flow. This work may be regarded as a
step forward to discuss extensions towards a novel DNS approach, describing the
mesoscale physics of small droplets subjected to a generic hydrodynamical
strain field, possibly mimicking the effect of a realistic turbulent flow on
dilute droplet suspensions
Universality in passively advected hydrodynamic fields: the case of a passive vector with pressure
Universality of statistical properties of passive quantities advected by
turbulent velocity fields at changing the passive forcing mechanism is
discussed. In particular, we concentrate on the statistical properties of an
hydrodynamic system with pressure. We present theoretical arguments and
preliminary numerical results which show that the fluxes of passive vector
field and of the velocity field have the same scaling behavior. By exploiting
such a property, we propose a way to compute the anomalous exponents of three
dimensional turbulent velocity fields. Our findings are in agreement within 5%
with experimental values of the anomalous exponents.Comment: 15 pages, 6 figure
Universality of anisotropic fluctuations from numerical simulations of turbulent flows
We present new results from a direct numerical simulation of a three
dimensional homogeneous Rayleigh-Benard system (HRB), i.e. a convective cell
with an imposed linear mean temperature profile along the vertical direction.
We measure the SO(3)-decomposition of both velocity structure functions and
buoyancy terms. We give a dimensional prediction for the values of the
anisotropic scaling exponents in this Rayleigh-Benard systems. Measured scaling
does not follow dimensional estimate, while a better agreement can be found
with the anisotropic scaling of a different system, the random-Kolmogorov-flow
(RKF). Our findings support the conclusion that scaling properties of
anisotropic fluctuations are universal, i.e. independent of the forcing
mechanism sustaining the turbulent flow.Comment: 4 pages, 3 figure
Shear-Improved Smagorinsky Model for Large-Eddy Simulation of Wall-Bounded Turbulent Flows
A shear-improved Smagorinsky model is introduced based on recent results
concerning shear effects in wall-bounded turbulence by Toschi et al. (2000).
The Smagorinsky eddy-viscosity is modified subtracting the magnitude of the
mean shear from the magnitude of the instantaneous resolved strain-rate tensor.
This subgrid-scale model is tested in large-eddy simulations of plane-channel
flows at two different Reynolds numbers. First comparisons with the dynamic
Smagorinsky model and direct numerical simulations, including mean velocity,
turbulent kinetic energy and Reynolds stress profiles, are shown to be
extremely satisfactory. The proposed model, in addition of being physically
sound, has a low computational cost and possesses a high potentiality of
generalization to more complex non-homogeneous turbulent flows.Comment: 10 pages, 6 figures, added some reference
Copepods encounter rates from a model of escape jump behaviour in turbulence
A key ecological parameter for planktonic copepods studies is their
interspecies encounter rate which is driven by their behaviour and is strongly
influenced by turbulence of the surrounding environment. A distinctive feature
of copepods motility is their ability to perform quick displacements, often
dubbed jumps, by means of powerful swimming strokes. Such a reaction has been
associated to an escape behaviour from flow disturbances due to predators or
other external dangers. In the present study, the encounter rate of copepods in
a developed turbulent flow with intensity comparable to the one found in
copepods' habitat is numerically investigated. This is done by means of a
Lagrangian copepod (LC) model that mimics the jump escape reaction behaviour
from localised high-shear rate fluctuations in the turbulent flows. Our
analysis shows that the encounter rate for copepods of typical perception
radius of ~ {\eta}, where {\eta} is the dissipative scale of turbulence, can be
increased by a factor up to ~ 100 compared to the one experienced by passively
transported fluid tracers. Furthermore, we address the effect of introducing in
the LC model a minimal waiting time between consecutive jumps. It is shown that
any encounter-rate enhancement is lost if such time goes beyond the dissipative
time-scale of turbulence, {\tau}_{\eta}. Because typically in the ocean {\eta}
~ 0.001m and {\tau}_{\eta} ~ 1s, this provides stringent constraints on the
turbulent-driven enhancement of encounter-rate due to a purely mechanical
induced escape reaction.Comment: 11 pages, 10 figure
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